AGU journal highlights -- Oct. 26, 2006

1. Manmade aerosols impact Southern Hemisphere oceanic circulation

Manmade aerosols (fine particles) tend to have a mitigating effect on greenhouse gas-induced climate warming because they scatter incoming solar radiation. Past studies of the impact of manmade aerosols on oceanic heat content have focused largely on globally averaged responses. Cai et al. noted that aerosol cooling is greater in the Northern Hemisphere than in the Southern Hemisphere and wondered whether aerosols induce a greater impact on oceanic heat content in the Northern Hemisphere. Through global climate model experiments forced with and without manmade aerosols, the authors showed that increasing aerosols in the20th century induced pan-oceanic heat redistribution. This involved a reduction in Southern Hemisphere oceanic heat content as heat was transported to Northern Hemisphere oceans. It included a strengthening in northward cross-equatorial heat transport in the Atlantic and Pacific oceans, with Atlantic currents associated with global oceanic overturning playing a major role. The authors note that as manmade emissions of aerosols decrease due to environmental regulations, the mitigating effect of this oceanic heat transport will decrease, exacerbating expected slowdowns of Atlantic Ocean overturning.

The lowermost layer of Earth's atmosphere, called the planetary boundary layer (PBL), is directly influenced by the presence of Earth's surface. Within this layer, frictional drag, solar heating, evapotranspiration, and other factors combine to create strong vertical mixing, which forms turbulent eddies of various sizes, some as thick as the PBL itself. Noting that weather models must consider mechanisms for PBL turbulence, Drobinski et al. sought to determine methods for characterizing eddies at fine scales, using energy and material flux measurements collected at the Site Instrumental de Recherche par Télédétection Atmosphéric (SIRTA) observatory in France. The authors found that atmospheric models perform well in the near-surface region and the free-stream turbulence region in the PBL when the standard Prandtl mixing length, a coarse length over which an air parcel can move and maintain its heterogeneity without being incorporated into the surrounding gases, is discarded. Instead, they suggest that models use a larger mixing length, found from heterogeneous turbulence data. They noted that their method improves the representation of PBL layer processes in most atmospheric stability conditions.

Clouds, which can reflect sunlight and absorb terrestrial radiation, are important components in the vertical distribution of energy. Variations in their occurrence, persistence, area, and altitude can affect the amount of solar and terrestrial radiation in the atmosphere and surface. Noting that clouds are the major modulators of energy in the tropics, Choi and Ho studied tropical cirrus clouds using the 1.38 micrometer reflectance band from the Moderate Resolution Imaging Spectroradiomenter (MODIS) data set and radiance information from the Clouds and the Earth's Radiant Energy System (CERES) data set. Using data collected between 2000 and 2005, they found that the cloud radiative effects of cirrus clouds followed predicable patterns, regardless of region or season. However, since a majority of tropical clouds are optically thin cirrus clouds, their net radiative effects are more than twice as strong as that of thick high clouds. This suggests that the often-ignored radiative effects of thin cirrus clouds should be monitored, because they have the potential to influence tropical energy balance calculations.

Title:
"Radiative effect of cirrus with different optical properties over the tropics in MODIS and CERES observations"

Emissions from trace gases are used to measure the temperature of the mesosphere and the lower thermosphere (MLT). The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite retrieves temperature in the MLT by measuring the infrared emissions of carbon dioxide. SABER temperature measurements in the mesopause (the boundary between the mesosphere and the thermosphere, the two highest layers of the atmosphere) during polar summer were found to be too low in both altitude and temperature when compared to results from other experiments. Kutepov et al. developed a method to more accurately convert SABER radiances to temperature profiles. They determined that in the very cold polar summer MLT, the transfer of molecular vibrational energy by collisions between carbon dioxide molecules of different isotopes has a significant impact on the infrared emission. Accounting for this effect shifts the retrieved 2002 summer polar mesopause altitude upward by two to four kilometers [around one to two miles]. It also increased the value of the temperature at the mesopause. This brings SABER temperature measurements into better agreement with outside data.

Severe weather conditions from hurricanes, such as large storm surges and devastating wind and rainfall along their tracks, make it difficult to perform detailed near-surface field observations of surface winds. However, spaceborne Synthetic Aperture Radar (SAR) can give detailed near-surface hurricane information. Shen et el. Have developed a new algorithm for retrieving hurricane wind speeds based on SAR data and the tendency of hurricanes to have radial symmetry in their surface wind structure. By assuming constant wind speeds and wind direction in three neighboring sub-image blocks of any specific concentric circle around the hurricane eye, the authors demonstrate that wind speed and wind direction can be resolved without any external information. Using a case study of 2003's Hurricane Isabel, wind speeds retrieved through their method were found to be consistent with outside satellite wind retrievals and on site buoy data. The authors note that since their new method does not rely on any external wind information, fine-resolution wind structures can be retrieved without introducing interpolation errors into calculations.

The high-latitude ionosphere is an important source of Earth's magnetospheric plasma. However, the transport of ionospheric ions into the magnetosphere is poorly understood. Yizengaw et al. analyzed an ionospheric outflow event that occurred on 16 December 2003, using data from Global Positioning System (GPS) total electron content (TEC) monitoring instruments, the magnetometer aboard the FedSat satellite, and ion drift meters aboard the Defense Meteorological Satellite Program satellites. With this data, the authors created a tomographic image, which is a cross-section that maps regions of varying density, of this ion outflow event. The image revealed a long beam-like structure extending to Earth's outer-magnetosphere. The authors note that their data can be used to better model ionospheric and plasmaspheric densities, which are important parameters for forecasting whether geomagnetic storms will disrupt communications and navigation systems.

I. Highlights, including authors and their institutions
II. Ordering information for science writers and general public

I. Highlights, including authors and their institutions

The following highlights summarize research papers in Geophysical Research Letters (GRL).

You may read the scientific abstract for any of these papers by
going to http://www.agu.org/pubs/search_options.shtml and
inserting into the search engine the portion of the doi (digital object
identifier) following 10.1029/ (e.g., 2006GL987654). The doi is
found at the end of each Highlight, below. To obtain the full text of
the research paper, see Part II.

II. Ordering information for science writers and general public

Journalists and public information officers of educational and
scientific institutions (only) may receive one or more of the papers
cited in the Highlights by sending a message to Jonathan Lifland
[[email protected]], indicating which one(s). Include
your name, the name of your publication, and your phone number.
The papers will be e-mailed as pdf attachments.

Others may purchase a copy of the paper online for nine dollars:

1. Copy the portion of the digital object identifier (doi) of the paper
following "10.1029/" (found under "Source" at the end of each
Highlight).